I know little about this, but having said that could a very high burst produce affect elements that are mostly stable against the normal background radiation le

I know little about this, but having said that could a very high burst produce affect elements that are mostly stable against the normal background radiation levels, but then decay into other elements when exposed to particular levels of gamma rays? So while general "rocks" might not have clear data, detecting the signatures of particular ratios of elements outside the normal baseline in a certain type of geological formation might provide evidence. Locating similar formations around the world could even triangulate to which side of the planet was hit.